On the MOTION of LIGHT. 87 



by moving furfaces, and briefly mention fome of the moft ufe- 

 ful corollaries. 



Let O, (fig. I.) be a terreflrial objedl, whofe pofition is to 

 be determined by an obferver placed at B', and furniflied with 

 a common furveyor's theodolite. He muft place the index of 

 his inflrument fo that the light coming from the objecSt in the 

 dire<5lion OB', may pais through the two holes A', B', which 

 conftitute the plane fights of his theodolite ; that is, the three 

 points O, A', B', muft be in a ftraight line. The inftrument is 

 fo conftrufted that the ftraight line pafling through the holes 

 A', B', is parallel to a line NS drawn along the ruler or index 

 upon which the fights are placed, and the divifion upon the 

 arch, which is cut by the line NS, indicates the pofition of the 

 objedl. 



But now let us fuppofe that, by the motion of the earth, the 

 obferver is uniformly carried along the ftraight line B'/3 perpen- 

 dicular to BO, while, in the mean time, the obje(5l is carried 

 with an equal motion along the Une OS' parallel to B'/3. It is 

 now evident, that, if the inftrument, without altering its po- 

 fition, be carried along in the direction B'/3, the light which 

 enters the hole A', in the direction OA', will not pafs through 

 the hole B ; for, when the light entered the hole A', the other 

 hole was at B ; but when the light arrives at B', the hole B' has 

 got to fome place R in the line B',S. Take the point B, fo that 

 the ftraight line 0;8 may be to B'/3 as the velocity of light to 

 the velocity of the earth. Through A' draw the ftraight line 

 A'Aa, parallel to B'/3, cutting the line 0/3 in the point A. Draw 

 DAB parallel to OA'B'. Now, let the theodolite move from the 

 fituation AB' to the new fituation AB, while the light moves 

 from O to A. It is evident that the light will enter the hole A, 

 and proceed in the direiflion AjS. In the mean time, the hole 

 B will have moved from-B to /3 ; for A/3 is to B/3, as O^ is to 

 B/3, that is, as the velocity of light to the velocity of the theo- 

 dolite. 



